Spot testing



United States Patent SPOT TESTING PaulW. McConnaughey, Wilkinsburg, Pa., assignor to Mine Safety Appliances Company, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application May 24, 1956 Serial No. 586,934

5 Claims. (Cl. 23-230) This invention relates to a method for the colorimetric determination of substances by spot testing, and especially for detecting such substances as lead compounds and fluorides deposited on a filter or similar material.

In present practice of spot testing, a drop of a solution containing a colorimetric reagent is deposited on the material under test. Most of the solvent evaporates at the periphery of the wetted area resulting in increased concentration of the reagent at the periphery and an ac companying peripheral intensity of color. In some instances the solvent may be absorbed by the filter and carry reagent with it, whereupon the resulting stain must be held to a light for proper evaluation for the nonuniformity of the stain is 'ditficult to compare with standards. Further, when the solvent must evaporate before the indicator will react, the lapsed time may militate against use of the specific test. As a consequence of such problems, it is not surprising that adequate determinations are difiicult to obtain.

It is an object of the present invention to provide a colorimetric spot testing method that is easy to use, that results in the characteristic color promptly, and that results in a color indication that is uniform.

It is another object to provide a method for the determination of lead with tetrahydroxyquinone (THQ) in which the color develops promptly, and in which the THQ is employed in a form having extended shelf life.

These objects are attained, in one embodiment, by supplying a colorimetric reagent as a suspension, in a liquid that is not a solvent for the reagent, to a filter, orother inert carrier containing the determinable constituent. Thus, I suspend a colorimetric reagent in a suitable medium and deposit the reagent on the filter as a suspension. Upon contact with the filter, the reagent is physically retained thereby resulting in a uniform deposit, for it does not migrate to the periphery of the wetted area or become absorbed by the filter.

The liquids that can be employed in the invention must be non-solvents for the reagent deposited and may not deleteriously interfere with the color-determination being made as by masking the characteristic color. Upon depositionof the suspension, the reagent will be physically retained by the filter while the liquid diffuses or migrates through the filter. Obviously, the particular liquid chosen for any determination is dependent upon the colorimetric reagent involved, for liquids that are satisfactory for one reagent may act as a solvent for the reagent of some other determination. As long as suflicient of the reagent is present in each drop of suspension to develop the color characteristic of the deposit, the concentration of reagent in the suspension is not of critical importance. In general, very satisfactory results have been obtained with suspension concentrations of the order of about one weight percent.

A characteristic that is'desirable in a colorimetric reagent is extended shelf life, for otherwise the practicality of the test may be limited by having the reagent components at hand when a test need be made. Thrs con- 2 sideration may influence the choice of liquid or the form in which the reagent is employed. For example, tetrahydroxyquinone is a specific reagent for the determination of lead, under sufliciently acid conditions; however its stability is limited. Consequently, this reagent should be used in a form having satisfactory shelf stability, and the liquid employed to produce a suspension thereof must be a non-solvent for the particular form used rather than for the active reagent as such. In the present invention, I employ tetrahydroxyquinone as a stable salt, such as disodium tetrahydroxyquinone, and disperse the salt in a liquid, such as ethanol. When a test is to be made, a drop of a buffer is added to release the THQ and render the spot sufiiciently acid for adequate results in this particular determination. I prefer to use a buffer solution of about pH 2.79; a standard Coleman butter tablet (pH 2.80) in 100 ml. of distilled water is suitable.

The invention will be illustrated by the following examples:

Example I A drop of one Weight percent suspension of disodiunr tetrahydroxyquinone in ethanol volume percent) was placed on a fiberglass filter paper known to contain lead. A drop of a pH 2.79 buffer was then deposited on the reagent to release THQ from the salt. A definite, uniform, red stain resulted.

In other tests under similar conditions, with regard to reagent concentrations, pH of the buffer and quantities of materials employed, a definite, uniform, red stain resulted with lead quantities as small as 0.3 microgram on an area of one square centimeter of the fiberglass filter paper.

Example 11 A suspension of zirconium azophenylarsonate, a hydrogen fluoride colorimetric reagent, was prepared by mixing ten parts by weight of a solution of 0.1 g. zirconium oxychloride in cc. of one normal hydrochloric acid (HCl) with one part of a solution of 0.5 gm. diphenylaminoazo arsonic acid in 45 cc. of ethyl alcohol containing 5 cc. of concentrated HCl and 17 cc. of glycerin. The glycerin was added to obtain finer particles in the suspension. A drop of the suspension was deposited on a fiberglass filter known to contain hydrogen fluoride. The resulting spot developed a stain which was uniform throughout the area of deposition.

As a consequence of this invention, colorimetric determinations can be made promptly with a high degree of accuracy. The use of a colorimetric reagent as a suspension presents a high and uniform surface area of the reagent for reaction with the determinable constituent;

contribute to speed of determination. The uniformity of the resultant stains contributes to ready and accurate comparison with standards.

According to the provisions of the patent statutes, I have explained the principle of my invention and have described what I now believe to be its best. embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

I claim: I

1. A method of spot testing comprising depositing on a carrier a colorimetric reagent as a suspension in a liquid that is not a solvent for said reagent, whereby a uniform deposit of reagent is obtained, and developing a color characteristic of the test being conducted by reacting said reagent as thus deposited with material under test.

2. That method of colorimetrically determining lead on an inert porous material comprising depositing a salt w a H-rw W ethanol and said buffer has a pH of about 2.8.

5. That method of colorimetric-ally determining hydroliquid on said porous material for reaction with the hydrogen fluoride.

References Cited in the file of this patent gen fluoride on an inert porousrmaterial, comprising de- 10 2,686,770

positing zirconium azophenylarsonate suspended in a UNITED STATES PATENTS Antrim Sept. 3, 1940 Kugler et al Mar. 13, 1951 Wilson June 23, 1953 Dipner Aug. 17, 1954 

1. A METHOD OF SPOT TESTING COMPRISING DEPOSITING ON A CARRIER A COLORIMETRIC REAGENT AS A SUSPENSION IN A LIQUID THAT IS NOT A SOLVENT FOR SAID REAGENT, WHEREBY A UNIFORM DEPOSIT OF REAGENT IS OBTAINED, AND DEVRELOPING A COLOR CHARACTERISTIC OF THE TEST BEING CONDUCTED BY REACTING SAID REAGENT AS THUS DEPOSITED WITH MATERIAL UNDER TEST. 